1. Field of the Invention
The present invention relates in general to pulse oximetry, and in particular to the processing of signals generated by a pulse oximeter.
2. Description of the Related Art
A pulse oximeter is typically used to measure various blood characteristics, including the blood oxygen saturation of hemoglobin in arterial blood and the pulse rate of the patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor that passes light through a portion of a patient's blood perfused tissue and photo-electrically senses the absorption and scattering of light in such tissue. The amount of light absorbed and scattered is then used to estimate the amount of blood constituent in the tissue using various algorithms known in the art. The “pulse” in pulse oximetry comes from the time varying amount of arterial blood in the tissue during a cardiac cycle. The signal processed from the sensed optical measurement is the familiar plethysmographic waveform, which corresponds with the cyclic attenuation of optical energy through a portion of a patient's blood perfused tissue.
Venous pulsation is an undesirable artifact in pulse oximetry. Venous pulsation is particularly common on the head or forehead, where the vascular anatomy lacks valves to prevent venous blood from backing up and pooling. Venous pulsation may be caused by the patient's medical condition, or during surgical interventions that interfere with venous return. The effects of venous pulsation may include: 1) oxygen saturation (e.g., SpO2) readings reflecting a mix of venous and arterial blood, which would be substantially lower than the arterial oxygen saturation, thus resulting in incorrectly low oxygen saturation measurements, and 2) pulse rate readings that are double or even triple the patient's pulse rate, due to the prominent harmonics in the venous pressure wave. In addition, in extreme cases, it is possible that an oximeter would fail to acquire oxygen saturation and/or pulse rate measurements. Unlike motion artifacts that may be intermittent, venous pulsation can continue uninterrupted for hours. While the side effects of venous pulsation are highly visible to the clinician, their cause may not be.